Patent GB-1512737-A (GB)
Title: Intra link calling within a subscriber digital multiplexing system
IPC: H04Q11/04
Filed: 1976-02-17
Abstract: 1512737 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 17 Feb 1976 [26 Feb 1975] 06146/76 Heading H4K In a telephone system comprising a central office, CO, connected via a TDM highway to a number of remote terminals A, B etc. to each of which are connected a plurality of subscribers lines, all connections are initially set up via and under the control of the central office, the arrangement being characterized in that in response to a called party going off-hook, a check is performed to determine whether the calling and called parties are connected to the same remote terminal, and if the check is positive the connection is transferred to a direct connection at the remote terminal, the connections via the central office then being released. The system is basically that of Specification 1,512,440 and a connection is established between two subscribers via the central office as disclosed in that Specification, i.e. a 32-channel highway is used to couple a maximum of 128 subscriber lines to the central office, the subscriber lines being arranged in groups of between 16 and 64 constituting remote terminals. For further details of the basic system reference is directed to the above Specification. To provide for a direction connection, referred to as an intra link (IL) connection, at a remote terminal extra circuitry comprising units 80, 82, 84 are provided at the central office. Basically the arrangement operates as follows. The IL control 84 includes a store for each of the 32 line channels of the highway extending to a set of remote terminals, and stores during a normal call set up procedure a condition indicating whether a time channel is being used for an originating or terminating call. For an originating channel a two minute timer is set on call initiation, it being assumed that any transfer to an intra link connection will be completed within this time, and the storage location associated with the channel has a bit position X reset after this period. The addresses of the originating and terminating subscriber lines are also stored in the IL store, it being determinable from the addresses whether or not the lines are connected to the same terminal. Hence when a called subscriber goes off-hook, the IL store is checked to see if any originating calls are taking place from the same remote terminal (i.e. bit position X, not reset). If there are no originating calls the connection remains set up via the central office. If, however, there are one or more originating calls, a digital tone generator is activated to supply a tone to the called time slot. This tone is eventually received back at the IL detector 80 in one of the originating channels, and the IL detector supplies the identity of the channel to the IL control. A second tone of a different frequency is then sent to the terminating channel and the operation repeated. A successful check causes the identities of the originating and terminating subscribers to be transferred from TDS memory 34 to IL memory 82 and thence to remote TDS memory 64 at the remote terminal to cause a direct connection to be set up in the remote terminal. If all locations of the IL memory 82 relating to one remote terminal are in use, thereby indicating that all the IL channels are in use, the call continues via the central office. Full details of the units 80, 82, 84 are given in Figures 2-9 (not shown). LED's are used in the IL memory 82 of Figure 6C to inidicate if all the IL connections of a group have been assigned. Transistors and diodes are employed in the IL detector Figure 7B.